1. Field of the Invention
The present invention generally relates to computer-implemented methods for generating input for a simulation program or generating a simulated image of a reticle. Certain embodiments relate to a computer-implemented method that includes combining information about a defect detected on a partially fabricated reticle with information about phase assigned to an area of the reticle proximate to the defect.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Semiconductor fabrication processes typically involve a number of lithography steps to form various features and multiple levels of a semiconductor device. Lithography involves transferring a pattern to a resist formed on a semiconductor substrate, which may be commonly referred to as a wafer. A reticle, or a mask, may be disposed above the resist and may have substantially transparent regions and substantially opaque regions configured in a pattern that may be transferred to the resist. For example, substantially opaque regions of the reticle may protect underlying regions of the resist from exposure to an energy source. The resist may, therefore, be patterned by selectively exposing regions of the resist to an energy source such as ultraviolet light, a beam of electrons, or an x-ray source. The patterned resist may then be used to mask underlying layers in subsequent semiconductor fabrication processes such as ion implantation and etch. Therefore, a resist may substantially inhibit an underlying layer such as a dielectric material or the semiconductor substrate from implantation of ions or removal by etch.
There are several types of reticles that are commercially available. For example, a binary reticle is a reticle having patterned areas that are either transparent or opaque. Binary reticles are different from phase-shift masks (PSM), one type of which may include films that only partially transmit light, and these reticles may be commonly referred to as halftone or embedded phase-shift reticles. If a phase-shifting material is placed on alternating clear spaces of a reticle, the reticle is referred to as an alternating PSM, an ALT PSM, or even a Levenson PSM. One type of phase-shifting material that is applied to arbitrary layout patterns is referred to as an attenuated or halftone PSM, which may be fabricated by replacing the opaque material with a partially transmissive or “halftone” film. A ternary attenuated PSM is an attenuated PSM that includes completely opaque features as well. Each of the reticles described above may also include a pellicle, which is an optically transparent membrane that seals off the reticle surface from airborne particulates and other forms of contamination.
A process for manufacturing a reticle is similar to a wafer patterning process. For example, the goal of reticle manufacturing is forming a pattern in an opaque material such as a relatively thin chrome layer on a substantially transparent substrate such as glass. In particular, reticle manufacturing may include a number of different steps such as pattern generation, which may include moving a glass substrate having a chrome layer and a resist layer formed thereon under a light source as shutters are moved and opened to allow precisely shaped patterns of light to shine onto the resist thereby creating the desired pattern.
Alternatively, reticles may be made with laser or e-beam direct write exposure. Laser exposure allows the use of standard optical resists and is faster than e-beam direct write exposure. In addition, laser systems are also less expensive to purchase and operate. Direct write laser sources are turned on and off with an acousto-optical modulator (AOM) or a digital multi-mirror. An example of a commercially available direct write laser system is the ALTA 3000® laser writer from Applied Materials, Inc., Santa Clara, Calif. Direct write e-beam systems are often used to manufacture complex reticles since they produce finer line resolution than laser systems. Examples of commercially available direct write e-beam systems include the MEBES 4500 and 5000 systems from Applied Materials. Other exposure types are also possible such as raster scan e-beam systems, vector scan e-beam systems, and quasi vector/raster scan e-beam systems.
After the exposure steps, the reticle is processed through development, inspection, etch, strip, and inspection. Defects in reticles are a source of yield reduction in integrated circuit manufacturing. Therefore, inspection of a reticle is a critical step in the reticle manufacturing process. As minimum pattern sizes shrink and integrated circuits are designed with higher device densities, defects that were once tolerable may no longer be acceptable. For example, a single defect may be repeated in each die in stepper systems and may destroy every die in single-die reduction reticles. In addition, due to the critical dimension (CD) budget of VLSI and ULSI-level integrated circuit manufacturing, the CD budget allowed for reticles requires substantially defect-free and dimensionally perfect reticles. For example, the overall CD budget for such integrated circuits may be approximately 10% or better thereby resulting in a CD budget for a reticle with about a 4% error margin.
Defects may be a result of incorrect designing of the reticle pattern and/or flaws introduced into the patterns during the pattern generation process. Even if the design is correct, and the pattern generation process is performed satisfactorily, defects in the reticle may be generated by the reticle fabrication process as well as during subsequent processing and handling. In addition to the many potential causes of defects, there are also many different types of defects. For example, bubbles, scratches, pits, and fractures may be a result of a faulty raw glass substrate. Defects in the opaque material may include particulate inclusions in the material, pinholes or voids in the material surface, and invisible chemical anomalies such as nitrides or carbides that may lead to erratic local etching and undesired patterns. Defects such as voids in the resist layer may produce pinholes that may lead to voids in the attenuating film. In addition, localized characteristics in the resist may also produce variations in characteristics of the resist such as resist solubility across the reticle substrate. Particulate matter may also be introduced to the reticle during processing and/or handling of the reticle. Defects that may result in inoperative devices or which would cause a die to be rejected at final wafer inspection are commonly referred to as “fatal” or “killer” defects, while others may be commonly referred to as “nonfatal” defects.
There are several methods that have been used to inspect reticles for defects. One method includes inspecting and repairing every defect detected after the first patterning/processing step. If too many defects are detected, the reticle is rejected (i.e., scrapped). In this method, no or little consideration is given to the defect's printability since only partial knowledge exists about the defect's final optical nature. Therefore, this method does not take into consideration the lithographic significance of individual defects in the reticle dispositioning decisions. Another method includes inspecting and repairing everything detected after the second or final patterning/processing step. In this method, if too many or non-repairable defects are detected, the reticle is rejected. Since phase defect repair is very difficult and/or expensive and/or has a significant impact on cycle time, in this method there is a greater likelihood of rejection of the completed mask/reticle.
In a different method, inspection is performed after the second or final patterning/processing step, and the impact of the defects on a printed image of the reticle is determined using an aerial image analysis tool. Only lithographically significant defects are repaired. As described above, since phase defect repair is very difficult and/or expensive and/or has a significant impact on cycle time, there is a greater likelihood of rejection of the completed mask/reticle. In another method, a reticle may be inspected using an aerial image inspection tool after the second or final patterning/processing step, and the detected defects may be repaired. By definition, these defects should be the lithographically significant defects. However, this method assumes that an aerial image inspection tool with sufficient sensitivity can be obtained to meet the A critical dimension (ΔCD) defect criteria. The major disadvantage of this method is that all patterning and processing steps need to be completed in order to make use of an aerial image inspection tool thereby requiring investment of the full cycle time and expense.
Accordingly, it may be desirable to develop a method for inspecting and/or evaluating defects on a reticle that eliminates one or more of the disadvantages of the methods described above.